I have an existing working site with lots of content. I've come to the realisation that I could actually sell this content. So is there a way to turn my existing content type into a product display for Drupal 7 Commerce or would you recommend to export all these nodes and create a new product display type and add them back in?
All you need is a product reference on a node for that node to be a "Display Node." A simple solution would be to just add that and any other fields you think necessary to the "selling" part of the node's function. You might need an alternative View Mode for the content type if there are items you need displayed or not dependent on circumstances.